1. Field of the Invention
This invention relates to a fuel injector, and particularly to a pressure accumulator type fuel injector having superior responsiveness of fuel pressure control.
2. Prior Art
To improve engine performance over a wide engine operating range extending from low speeds to high speeds, diesel engines have been fitted with pressure accumulator type fuel injection systems (common rail systems) capable of supplying high-pressure fuel stored in an accumulator in a stable manner to each cylinder of the engine.
However, even when this kind of fuel injection system is used, sudden explosive combustion occurs near the start of combustion when the fuel injection rate immediately after the start of fuel injection in the fuel injection cycle is excessively large, which results not only in an increase in engine noise, but also in an increase in the amount of oxides of nitrogen (NOx) in the exhaust gas.
To eliminate this kind of problem, common rail fuel injection systems which start fuel injection at a somewhat low fuel injection rate in an initial stage of the fuel injection cycle have been proposed.
As shown in FIG. 7, this kind of common rail fuel injection system 1 has a high-pressure common rail 2 storing high-pressure fuel pressurized by a fuel pump 1a, a low-pressure common rail 7 storing fuel at a lower pressure than the high-pressure fuel in the high-pressure common rail 2, and a fuel injection valve 3 for injecting high-pressure fuel from the high-pressure common rail 2 and low-pressure fuel from the low-pressure common rail 7 into a combustion chamber of the engine. The fuel injection valve 3 is connected to the high-pressure common rail 2 by a fuel supply pipe 11. The fuel injection valve 3 has inside it a pressure control chamber 3a and a fuel chamber 3b connected to the fuel supply pipe 11, and has a closing valve 15 for fuel injection timing control interposed between the pressure control chamber 3a and a fuel discharge conduit (not shown).
A pressure-switching valve 4 is provided in the fuel supply pipe 11, and a branch fuel pipe 12 branches from the fuel supply pipe 11 on the downstream side of this pressure-switching valve 4. The branch fuel pipe 12 includes two mutually parallel conduit sections, in one of which an orifice 5 is disposed and in the other of which a check valve 6 is disposed, and the low-pressure common rail 7 is connected to the fuel supply pipe 11 by this branch fuel pipe 12. Also, an electromagnetic pressure control valve 8 for controlling the fuel pressure of the low-pressure common rail 7 to a predetermined fuel pressure lower than that of the high-pressure fuel in the high-pressure common rail 2 is provided in a fuel return pipe 7a extending between the low-pressure common rail 7 and a fuel tank 10.
The check valve 6 thus has one side connected to the low-pressure common rail 7 and the other side connected by the branch fuel pipe 12 to the fuel supply pipe 11 downstream of the pressure-switching valve 4. As shown in FIG. 8, the check valve 6 has a valve member 17 received in a cylindrical housing 16 and a spring 18 urging the valve member 17 in its closing direction. This check valve 6 is constructed to open and allow a flow of fuel from the low-pressure common rail 7 to the fuel supply pipe 11 when the fuel pressure in the low-pressure common rail 7 rises above the sum of the fuel pressure in the fuel supply pipe 11 and the urging force of the spring 18. In FIG. 8, the reference numeral 19 denotes a spring receiving part and the reference numeral 16a a valve seat part.
In the fuel injection system of FIG. 7, until a fuel injection start time is reached, both the pressure-switching valve 4 and the closing valve 15 are kept closed, and the connection between the fuel injection valve 3 and the high-pressure common rail 2 and the connection between the pressure control chamber 3a and the fuel discharge conduit are cut off. In this state, due to the action of the orifice 5 and the check valve 6, the fuel in the fuel supply pipe 11 downstream of the pressure-switching valve 4 and the low-pressure fuel in the low-pressure common rail 7 are at the same pressure, and consequently low-pressure fuel from the fuel supply pipe 11 is supplied to the pressure control chamber 3a and the fuel chamber 3b of the fuel injection valve 3.
When the fuel injection start time is reached, the closing valve 15 is opened and fuel in the pressure control chamber 3a is discharged through the fuel discharge conduit. This causes a fuel pressure pushing a needle valve 3c in its closing direction to fall, and consequently the needle valve 3c is moved in its opening direction by the fuel pressure of the fuel chamber 3b against the urging force of a return spring 3d urging it in its closing direction, and the fuel injection valve 3 opens and a low-pressure initial injection (hereinafter, xe2x80x98low-pressure injectionxe2x80x99), wherein low-pressure fuel in the fuel chamber 3b is injected, is carried out. When the low-pressure injection period elapses, the pressure-switching valve 4 is opened and high-pressure fuel from the high-pressure common rail 2 is supplied through the fuel supply pipe 11 to the fuel chamber 3b, and a high-pressure main injection (hereinafter, xe2x80x98high-pressure injectionxe2x80x99), wherein high-pressure fuel is injected, is carried out following the low-pressure injection. Next, when an injection end time is reached, the closing valve 15 is closed and the connection between the pressure control chamber 3a and the fuel discharge conduit is cut off, the fuel pressure in the pressure control chamber 3a rises, the pushing force pushing the needle valve 3c in its closing direction increases, and the fuel injection valve 3 closes. Also, the pressure-switching valve 4 is closed, and high-pressure fuel in the fuel supply pipe 11 flows into the low-pressure common rail 7 through the orifice 5. When the fuel pressure in the low-pressure common rail 7 rises, the pressure control valve 8 is duty-controlled so that the fuel pressure in the low-pressure common rail 7 assumes a predetermined fuel pressure lower than that of the high-pressure fuel, and some of the fuel in the low-pressure common rail 7 is discharged to the fuel tank 10 as necessary.
In this way, the common rail fuel injection system 1 switches the fuel injection waveform from a low pressure to a high pressure by operating the pressure-switching valve 4 during the fuel injection period, i.e. the period for which the closing valve 15 is open, and in an initial stage of fuel injection, because a low-pressure injection is carried out, combustion is effected relatively slowly and the amount of NOx emissions in the exhaust gas is reduced. Also, because a high-pressure injection is being carried out at the end of fuel injection, the fuel injection rate falls rapidly as soon as the closing valve 15 closes, and the emission of smoke and particulates is reduced.
This common rail fuel injection system 1 of the related art includes the fuel supply pipe 11 extending between the fuel injection valve 3 and the pressure-switching valve 4 and the branch fuel pipe 12 extending between the fuel supply pipe 11 and the low-pressure common rail 7. Consequently, the fuel injection system 1 as a whole occupies a large space and has poor ease of mounting to the engine. In particular, when the fuel injection valve is to be mounted over the center of the combustion chamber on a four-valve/cylinder diesel engine having two intake valves and two exhaust valves per cylinder, the installation space for the fuel injector on the cylinder head 71 is narrow and it is essential for a fuel injector to be mounted on this kind of engine to be made compact.
Also, the lengths of the fuel supply pipe 11 and the branch fuel pipe 12 are long, and the internal volumes of the fuel supply pipe 11 and the branch fuel pipe 12 are large. Consequently, from when the pressure-switching valve 4 is opened to increase the injection pressure, it takes time for the fuel pressure in the fuel chamber 3b to rise from the low pressure to the high pressure, and also, from when the pressure-switching valve 4 is closed, it takes time for the fuel pressure in the fuel chamber 3b to reach the low pressure which is proper for the start of the next injection cycle. In other words, the passage sections shown with thick arrow lines in FIG. 7 constitute dead volume in fuel pressure control and impair the responsiveness of fuel pressure control.
One conceivable way of improving the responsiveness of fuel pressure control is to use an injection unit in which the fuel injection valve 3 and the pressure-switching valve 4 are integrated; however, in the fuel injection system 1 described above, high-pressure fuel passes through the pressure-switching valve 4 as the valve member of the pressure-switching valve 4 reciprocates, and when the fuel injection system 1 is used over a long period, the valve member and the valve seat of the pressure-switching valve 4 suffer wear and high-pressure fuel in the high-pressure common rail 2 tends to leak through the pressure-switching valve 4 to the fuel supply pipe 11 on the downstream side when the pressure-switching valve 4 is closed. That is, the pressure control function of the pressure-switching valve 4 is lost. In this case it is necessary for the pressure-switching valve 4 to be replaced, but in a fuel injection system wherein the fuel injection valve 3 and the pressure-switching valve 4 are simply integrated into an injection unit, even when there is no problem with the fuel injection valve 3, the whole injection unit has to be replaced, and labor time becomes long, the price of the replacement part becomes high, and as a result the repair cost is high.
Also, whereas, as has already been mentioned, it is desirable for the fuel flow passage volume of the fuel supply pipe 11 and the branch fuel pipe 12 extending between the fuel injection valve 3, the pressure-switching valve 4, the orifice 5 and the check valve 6, i.e., the dead volume of injection pressure control, to be minimized, in the check valve 6 of the related art fuel injection system 1, as shown in FIG. 8, the flow passage area of the spring receiving part 19 is considerably larger than the flow passage area of the valve seat part 16a, and thus the spring receiving part 19 constitutes another dead volume.
It is therefore an object of the present invention to provide a fuel injector having superior responsiveness of fuel pressure control and ease of mounting to an engine.
To achieve the above-mentioned object and other objects, a first fuel injector provided by the invention comprises a housing, mounted to a cylinder head of an engine; a fuel injection valve, fitted in a first end of the housing and having a first control valve for controlling fuel injection to a combustion chamber of the engine; a first conduit, formed in the housing and having one end connecting with the fuel injection valve and its other end opening in a second end of the housing and connected to a high-pressure fuel source storing high-pressure fuel; a second control valve, fitted to the housing, for controlling a supply of high-pressure fuel to the fuel injection valve by opening and closing the first conduit; a second conduit, formed in the housing and having one end connecting with the first conduit downstream of the second control valve and its other end opening in the housing and connected to a low-pressure fuel source storing low-pressure fuel at a lower pressure than the fuel pressure of the high-pressure fuel source; and a check valve, fitted in the housing and disposed to the second conduit, for allowing the inflow of low-pressure fuel from the low-pressure fuel source to the first conduit side.
In this first fuel injector provided by the invention, the second control valve, the check valve and the fuel injection valve are mounted to a housing and essentially are integrated with each other. Because of this, the first conduit and the second conduit connecting together the second control valve, the check valve and the fuel injection valve are formed in the housing, thereby the lengths of the first conduit and the second conduit can be made short; the fuel flow passage volume affecting the responsiveness of fuel pressure control when a low-pressure injection and a high-pressure injection are selectively carried out, i.e. dead volume, can be greatly reduced; and the responsiveness of fuel pressure control can be greatly increased. Also, the fuel injector as a whole becomes compact, and the mountability of the fuel injector to an engine improves. In the mounting of the second control valve, the check valve and the fuel injection valve to the housing, the respective main parts of the second control valve, the check valve and the fuel injection valve can for example be received in three holes formed in the housing. Further, the second control valve, the check valve and the fuel injection valve can be connected together in the housing by the first conduit and the second conduit having short lengths.
In a second fuel injector provided by the invention, the housing is mounted on the cylinder head with the first end of the housing positioned substantially over the center of the combustion chamber and the second end of the housing positioned radially outward of the combustion chamber, and the fuel injection valve, the check valve and the second control valve are so fitted in the housing that they form a substantially straight line in order from the first end of the housing to the second end thereof.
In this second fuel injector, because the fuel injection valve, the check valve and the second control valve are so fitted in the housing that they form a substantially straight line in order from the first end of the housing to the second end thereof, the housing becomes more compact and particularly the width dimension of the housing decreases.
In a third fuel injector provided by the invention, the external diameter of the check valve is smaller than the external diameter of the fuel injection valve and the external diameter of the second control valve, and the check valve is so fitted in the housing as to be positioned between an intake valve and an exhaust valve of the engine.
In this third fuel injector, because the housing is mounted to the cylinder head with the small-diameter check valve disposed between the fuel injection valve and the second control valve and positioned between an intake valve and an exhaust valve, the narrow installation space between an intake valve and an exhaust valve can be effectively utilized to install the housing fitted with the check valve, the fuel injection valve and the second control valve to the cylinder head. And, in order that the small-diameter check valve can be positioned between an intake valve and an exhaust valve, the width of that part of the housing may be made smaller than that of the other parts of the housing where the fuel injection valve and the second control valve are fitted, to further improve the mountability of the housing to an engine.
In a fourth fuel injector provided by the invention, the engine is a four-valves/cylinder engine having a rocker shaft disposed on one side of the combustion chamber, an intake rocker arm and an exhaust rocker arm are rotatably supported on the rocker shaft, and a pair of intake valves and a pair of exhaust valves are respectively disposed to each cylinder; the housing is mounted on the cylinder head in a space between the pair of intake valves and the pair of exhaust valves of the combustion chamber; and the housing is mounted on the cylinder head with the first end of the housing positioned substantially over the center of the combustion chamber and the second end of the housing positioned on the opposite side of the combustion chamber from the rocker shaft.
In this fourth fuel injector, the narrow installation space between a pair of intake valves and a pair of exhaust valves can be effectively utilized to install the housing fitted with the check valve, the fuel injection valve and the second control valve to the cylinder head. And, because the outer end of the housing is disposed so as to be positioned on the opposite side of the combustion chamber from the rocker shaft, the housing can be disposed efficiently on the cylinder head and the mountability of the housing to the cylinder head can thus be further improved.
In an eighth fuel injector provided by the invention, having the same basic construction as the first fuel injector, the second control valve is removably fitted in the housing and the second control valve has a control valve member and a valve member fitted to the control valve member, the valve member for closing the first conduit under a piston action pressure impressed by some of the high-pressure fuel from the high-pressure fuel source, and, when the piston action pressure is opened to the atmosphere, cancels the closing of the first conduit by the valve member and allows the inflow of high-pressure fuel from the high-pressure fuel source to the first conduit.
In this eighth fuel injector, because the second control valve is removably fitted in the housing receiving the fuel injection valve, the fuel injection valve and the second control valve are essentially integrated, and the first conduit for connecting the two becomes short. Consequently, the dead volume of fuel pressure control decreases and the responsiveness of fuel pressure control is improved. Also, the second control valve can be removed from the housing and the whole second control valve can be replaced or the valve member or a control valve body of the second control valve can be replaced. Accordingly, when there is no problem with the fuel injection valve, it is only necessary to remove and replace the second control valve, and the maintenance cost of the fuel injector decreases. And, the second control valve is a differential pressure actuation type which opens and closes in accordance with whether or not a piston action pressure is being impressed on its valve member, and compared to one of a type which on/off-controls the high-pressure fuel directly it is small and compared to a spool valve or the like it has less fuel leakage, and thus it contributes to making the fuel injector compact and to improving the precision of fuel pressure control.
In a ninth fuel injector provided by the invention, having the same basic construction as the first fuel injector, the check valve has a valve member, a spring urging the valve member in its closing direction, and a check valve body having a receiving space for receiving the valve member and the spring, and the check valve body has on an inner circumferential wall face thereof a convexity projecting toward the spring, and the convexity is so formed that, when the valve member moves toward the second conduit side against the urging force of the spring by an inflow of low-pressure fuel from the low-pressure fuel source to the second conduit, there remains an annular conduit between the convexity and the moved valve member.
In this ninth fuel injector, because a convexity is formed on the inner circumferential wall face of the check valve body, the volume of the space receiving the check valve, that is, the dead volume of fuel pressure control here, is smaller, and consequently the responsiveness of fuel pressure control is superior. In particular, the rise characteristic of the fuel pressure at the time of switching of the fuel pressure from a low pressure to a high pressure improves. As a result the freedom of injection rate waveform control increases, and this is effective in reducing exhaust gases.
Preferably the convexity is formed all the way around the inner circumferential wall face of the check valve, to achieve a maximal reduction in dead volume.
Also, in the ninth fuel injector, in carrying out fuel pressure control, during fuel injection the fuel pressure is switched from a low pressure to a high pressure by the second control valve being opened, and after fuel injection the fuel pressure in the fuel passages is lowered by the second control valve being closed. Here, when the second control valve is opened to switch from a low-pressure injection to a high-pressure injection, some high-pressure fuel flowing into the fuel passages from the high-pressure fuel source flows into the space receiving the check valve, and when the receiving space is filled with high-pressure fuel and the valve member of the check valve assumes its closed position this flow of high-pressure fuel ends.
Accordingly, in this fuel injector, because the volume of the space receiving the check valve, that is, the dead volume here, is small, the valve member of the check valve closes immediately after the second control valve opens. In other words, when the second control valve opens the fuel pressure in the first conduit and the second conduit rises rapidly, and consequently the rise gradient of the injection pressure from the low-pressure injection is large. This kind of fuel pressure rise characteristic particularly contributes an improvement of responsiveness pertaining to the switching from the low-pressure injection to the high-pressure injection.
In a tenth fuel injector provided by the invention, the check valve has a throttle portion, and the throttle portion is so formed extending in the axial direction of the valve member as to allow restrictively an inflow of fuel from the first conduit side to the low-pressure fuel source side.
In this tenth fuel injector, because after the fuel injection of a fuel injection cycle high-pressure fuel remaining in the fuel passage is fed back to the low-pressure fuel source through a throttle of the check valve and provides a fuel pressure, low-pressure fuel can be stored without the low-pressure fuel source being provided with pressurizing means.